But behind the scenes, the president soon began entertaining the idea of firing Mr. Mueller even as his staff tried to discourage him from something they believed would turn a bad situation into a catastrophe, according to several people with direct knowledge of Mr. Trump’s interactions. A longtime friend, Christopher Ruddy, surfaced the president’s thinking in a television interview Monday night, setting off a frenzied day of speculation that he would go through with it.

For now, the staff has prevailed. “While the president has every right to” fire Mr. Mueller, “he has no intention to do so,” the White House spokeswoman Sarah Huckabee Sanders told reporters late Tuesday.

.. But people close to Mr. Trump say he is so volatile they cannot be sure that he will not change his mind about Mr. Mueller if he finds out anything to lead him to believe the investigation has been compromised.

.. his ability to endure a free-ranging investigation, directed by Mr. Mueller, that could raise questions about the legitimacy of his Electoral College victory, the topic that most provokes his rage

.. In his testimony before the Senate Intelligence Committee later in the day, Attorney General Jeff Sessions refused to answer what he said was a hypothetical question of whether he would support Mr. Mueller.

.. The president was pleased by the ambiguity of his position on Mr. Mueller, and thinks the possibility of being fired will focus the veteran prosecutor on delivering what the president desires most: a blanket public exoneration.

.. For Mr. Trump, the line between whim and will is always thin. It is often erased in moments of anger, when simmering grievance boils over into rash action ..

.. after a weekend of brooding

.. Mr. Trump was especially outraged by Mr. Comey’s admission last week that he had leaked a memo with details of his interactions with the president in hopes of spurring the appointment of a special counsel.

.. Among the aides most alarmed by the idea of firing Mr. Mueller, according to people familiar with the situation, was Reince Priebus

.. Donald F. McGahn II, the White House counsel, has also advised against firing Mr. Mueller.

.. Melania, has adopted a more temperate tone, telling her husband that she believed the appointment of Mr. Mueller would speed resolution of the Russia scandal and expressing her view that he would be exonerated

.. Mr. Ruddy has told friends that he went public with the Mueller story, in part, to prevent Mr. Trump from making a rash decision.